1. Field of the Invention
The present invention relates to legumes which contain high concentrations of zinc in the grain, and are therefore useful as foods and as foodstuffs, and a method for producing the same.
2. Description of the Related Art
In order for a human being to sustain life, trace metal elements such as iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), cobalt (Co), molybdenum (Mo), vanadium (V), selenium (Se), chromium (Cr), nickel (Ni), iodine (I), silicon (Si), fluorine (F), arsenic (As) and lead (Pb) need to be taken in from external sources, and these elements are called essential elements (Sakurai and Tanaka, eds. (1993), Bioinorganic Chemistry, Hirokawa-book-store). Among them, zinc (Zn) occupies the second largest portion of the trace metal elements contained in the human body, next to iron. Zinc is contained in important enzymes such as carboxypeptidases, carboxylate dehydrogenases and alcohol dehydrogenases, and is known to be responsible for roles important in the metabolic system of the body. It is also known that if zinc is deficient, growth disorder, decreased sexual function, impairment of the skin or hair, taste disturbance and the like are exhibited, thus causing problems. Moreover, while the daily zinc requirement for adult is known to be 12 to 15 mg, it is assumed that only about 9 mg of zinc per day can be taken in from an average Japanese meal. Thus, zinc deficiency in Japanese people has been regarded as a social issue (Tomita (1998), Secrets of Mineral Zinc Power for Attaining Health, Ohzora Publishing, Co.). In this regard, measures have been taken by public agencies, such as that in 2002, the Ministry of Health, Labor and Welfare in Japan added zinc as a component of the food with nutrient function claims, and in 2004, the Ministry of Education, Culture, Sports, Science and Technology in Japan established a target level for the zinc content in school meals.
As discussed above, since zinc (Zn) is important for the human body to carry on vital activities, it is desired to take a proper amount of zinc from daily meals. However, foods containing these elements to high levels are relatively limited. For example, zinc (Zn) is contained in oyster and cow liver to high concentrations such as 13.2 mg/100 g and 3.8 mg/100 g, respectively (Kagawa (ed. supervisor) (2003), “Tables of Food Composition, Fifth Revised, 2003,” Kagawa Nutrition University Publishing Division). However, from the standpoint of the current eating habits of Japanese people, it cannot be said that daily intake of such foodstuffs is common. On the contrary, Japanese people have an eating habit of routinely ingesting legumes. For example, soybean can be said to be a foodstuff that can be comfortably taken in almost everyday, in the form of cooked beans as well as processed food products including soy sauce, soybean paste (miso), tofu, natto and the like. Also, cowpea, soybean, alfalfa and the like can also be comfortably taken in the form of sprouts. From this point of view, it is envisaged that it will be useful to increase the contents of these trace metal elements in legumes. However, in the technical field related to the method for cultivating legumes, although the research concerning trace metal elements is being conducted through the minimum necessary research on the method for fertilization or the like, a sufficient level has not been reached regarding the technology of actively incorporating the trace metal elements into the edible parts.
Although rye does not belong to the legumes, recently there has been developed a technology of overexpressing Arabidopsis-derived zinc transporter gene through genetic engineering, to increase the zinc content in the grains of rye. However, even though this genetically engineered crop was fertilized with zinc, the zinc absorption rate was not elevated (Ramesh, et al. (2004), Plant Mol. Biol). The reason for this is contemplated to be because although zinc transporter gene is expressed, in the presence of zinc, the zinc transporter seems to be perceived as another metal transporter protein (Connoly, et al. (2002), Plant Cell), and thus the zinc transporter protein is eliminated by post-translational regulation. As such, despite the use of the genetic engineering technology which is a leading-edge technology at present, it is still difficult to make trace metal elements such as zinc to be transported into the edible part of crops.
Furthermore, as one of conventional fertilization techniques, a foliar spraying method has been used practically. Since this method is capable of incorporating fertilizer components into the cells which are brought into contact with the spray liquid, the method has been said to prevent or ameliorate the element deficiency symptoms in the leaves. However, there has been no report on a technology of accumulating a high concentration of a metal element in the seeds, which have no chance to be brought into direct contact with the spray liquid, by translocating the metal element from the leaves to the seeds, that is, by transferring the metal elements through a plurality of cells. Particularly, since zinc is not an element which is likely to be translocated, such as nitrogen, phosphorus, potassium or magnesium (Marschner (1995), Mineral Nutrition of Higher Plants, 2nd ed., Academic Press), it was difficult with the conventional foliar spraying method to accumulate zinc in the seeds to a high concentration.